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Who does IH affect?
The pathogenesis of IH is unknown. An autosomal dominant mode of inheritance has been considered by researchers because around 50% of people with IH have at least one family member who also has symptoms. It appears IH may be more common in females. The prevalence of IH is thought to be 0.005%–0.3% but, due to lack of epidemiological studies, biological markers and misdiagnosis of the disease, the real prevalence is unclear.

Though the symptoms mainly begin in adolescence or young adulthood (typically between 15-30 years of age), it is not uncommon for there to be a delay of many years before an accurate diagnosis is made.

More about IH
IH was defined by Czech neurologist Bedrich Roth more than 60 years ago* starting with his first monograph; "Narcolepsy and hypersomnia from the aspect of physiology of sleep (Narkolepsie a Hypersomnie S. Hlediska Fysiologie Spanku – 1957)" . Roth’s years of extensive research that led to his description of idiopathic hypersomnia as a separate disease entity was accepted and included in the first ICSD (International Classification of Sleep Disorders), the Diagnostic Classification of Sleep and Arousal Disorders in 1979. Since then it has been recognised as a “Rare Disease” and is included in the Genetic and Rare Diseases Information Center (GARD) register and Orphanet.

Bedrich Roth coined the term "Idiopathic Hypersomnia" in 1976 and defined two forms of the disease, Monosymptomatic and Polysymptomatic. While the terms polysymptomatic and monosymptomatic are no longer used research [1] supports the findings of previous studies [2,3,4,5] that show there are at least two forms of Idiopathic Hypersomnia. These studies show a subgroup of patients with “a complete form” of Idiopathic Hypersomnia with symptoms that are unique to this group ('complete' means they typically experience all of the key symptoms of Idiopathic Hypersomnia) [1]. This group is often referred to as Idiopathic Hypersomnia "with long sleep".

The research suggests that ‘complete’ Idiopathic Hypersomnia is, in fact, a separate sleep disorder of ‘incomplete’ Idiopathic Hypersomnia (previously referred to as ‘without long sleep’). In 2020, 21 of the world's Narcolepsy and Idiopathic Hypersomnia researchers wrote in two papers [6,7] that the complete form of Idiopathic Hypersomnia (with long sleep) should be recognised as an independent clinic entity (a medical disorder separate from Idiopathic Hypersomnia without long sleep). As a result of the overwhelming research [8,9,10] both groups also support the merging of Narcolepsy Type 2 (without cataplexy) and Idiopathic Hypersomnia without long sleep into one group. The majority suggest renaming this group "Idiopathic excessive sleepiness" to reflect the primary symptom of the group [6].

Therefore there would be three diagnostic categories:
Narcolepsy = Narcolepsy with Hypocretin/Orexin deficiency (with cataplexy)
Idiopathic Hypersomnia = with long sleep/the complete form of Idiopathic Hypersomnia
Idiopathic Excessive Sleepiness = Those currently diagnosed and who fall under the current definition of with Narcolepsy type 2 (without cataplexy) and Idiopathic Hypersomnia without long sleep (incomplete Idiopathic Hypersomnia)

*More about the history of Bedrich Roth and narcolepsy and hypersomnia:
Bedřich Roth, His Life’s Work and the 35th anniversary of the book “Narcolepsy and Hypersomnia”

Bedřich Roth: pioneer in Sleep Medicine

Click here for our IH Factsheet* and click here to read a comprehensive review** of how we have gone from the identification of Idiopathic Hypersomnia to where we are now. This review is drawn from nearly 60 references, including at least 40 peer reviewed studies on Idiopathic Hypersomnia and Narcolepsy that span more than 6 decades as well as numerous personal conversations with the world's leading Idiopathic Hypersomnia researchers.
If you are treating patients with Narcolepsy Type 2 (without cataplexy) or are a patient yourself you may also be interested in this review.

If you are a medical professional and would like free copies of our trifold IH Information Brochure** for your practice please contact us.

*Our Factsheet/brochure was written by HA's director Michelle Chadwick and has been vetted and is endorsed by Professor Ron Grunstein, MBBS, MD, PhD, FRACP and international Idiopathic Hypersomnia researcher Professor Karel Šonka MD, DSc.
 

**Our comprehensive review was written in 2018. Two papers were published in 2020 that provide an update to our review. Please refer to references 6 and 7 below.